This invention relates to water-soluble resin compositions, to aqueous solutions of such resin compositions, and to a process for the preparation of aqueous solutions of water-soluble resins.
It is known that water-soluble derivatives of epoxy resins can be used in aqueous thermosetting coating systems for electrodeposition of coatings onto metals. GB-A-1,556,201, for example, discloses resinous compounds prepared by reacting in the temperature range of from 100.degree. to 150.degree. C. hydroxyl-containing resinous intermediates with sufficient polycarboxylic acid anhydride to produce a resinous compound having an acid value of at least 35 mg KOH/g. The resinous intermediate is prepared by reacting below 150.degree. C. a polyglycidyl ether of a dihydric phenol with a hydroxy-alkane monocarboxylic acid or a mixture thereof with a saturated aliphatic dicarboxylic acid having 4-12 carbon atoms per molecule, the intermediate being made essentially carboxyl-free by the choice of a proper equivalent ratio of components, the use of rather low temperatures, and the use of a catalyst promoting the esterification of epoxy groups with carboxyl groups. The final resinous compounds are then diluted with a lyotropic solvent such as a glycol ether, neutralized with a base and combined with a crosslinking resin. They can then be further diluted with water to make an aqueous solution of a thermosetting coating composition. The films obtained can be cured by action of heat.
Aqueous solutions of these known thermosetting coating compositions tend to deteriorate on storage, giving rise to deposits and corrosion and to inferior physical properties in cured coatings, such as low sterilization resistance, low pasteurization resistance and low solvent resistance. This deterioration is believed to be due at least in part to hydrolysis of ester linkages.
It is therefore an object of the invention to provide an aqueous solution of a resinous compound which has improved storage stability and provides cured coatings having excellent sterilization resistance, pasteurization resistance and solvent resistance.